Vision inspection systems are well known. Generally, vision inspection systems can be divided into two categories, laser-based vision inspection systems which utilize a reflected or coherent laser light source to scan an object being inspected, as shown for example in U.S. Pat. Nos. 4,297,587 and 4,972,091, and camera-based vision inspection systems which utilize a suitable light source other than a laser light source to illuminate an object being inspected, as shown for example in U.S. Pat. Nos. 4,118,730 and 4,559,603. The present invention is concerned only with camera-based vision inspection systems.
The most prevalent type of camera used for camera-based vision inspection systems is a charge coupled device (CCD) camera. In a CCD camera, a charge coupled device is used as the mechanism to read out the values measured by a scan array of light sensors. There are two different kinds of CCD cameras that are typically used with CCD camera-based vision inspection systems: area scan cameras and line scan cameras. Although both types of CCD cameras use the same electronics to generate a video signal, the primary difference between area scan cameras and line scan cameras is that each type of camera produces a different type of video output signal.
In both cases, the CCD camera produces a video output signal representative of the light received by the scan array of light sensors during a predetermined period of time referred to as the camera scan time. The video output signal from an area scan camera is a composite video signal that includes a front porch and back porch having encoded reference values, as well as a composite sync signal. The reference values are used to decode the information contained between the front porch and back porch of the composite video signal. The sync signal is used to synchronize a series of lines of composite video signals that will be combined together to represent the area being scanned. In contrast, the video output signal of a line scan camera has no composite sync signal because only a single line is being scanned, not an entire area. In addition, there are no front porch and back porch portions of the video output signal of a line scan camera, although the video output signal of a line scan camera does include a black reference value.
The information portion of the video output signal generated by a CCD camera consists of a voltage signal referred to as the pedestal. During a first portion of the camera scan time, light striking a photosite in the scan array will discharge or decrease a reference charge level to which the photosite has been precharged. During a second portion of the camera scan time, the charge level remaining at the photosite is transferred out of the scan array using the charge-coupled device read out mechanism. As a result, the pedestal is the difference between the reference charge level and the charge level that remains at the end of the first portion of the camera scan time. These differences in charge levels are representative of differences in light received by different photosites from different locations in the focal plane of the CCD camera.
Camera-based vision inspection systems are very useful for detecting defects in objects, particularly in the surfaces of objects and defects contained in the base material such as dirt particles or holes by detecting and analyzing the deviations in the video output signal as compared to an expected video output signal if the object had no defects. U.S. Pat. Nos. 4,403,294, 4,509,076 and 4,974,261, and European Patent Appl. 0 543 629 A1, all show defect detection systems that utilize an area scan camera to detect defects in the surface of a stationary object. U.S. Pat. Nos. 4,240,110, 4,951,223 and 5,118,195 show vision inspection systems that utilize an area scan camera to detect defects in moving objects, particularly elongated strips or webs of material. Because the movement of the web relative to the camera allows a line scan camera to scan the entire web using only a successive series of line scans, and because the video output signal of a line scan camera is relatively simpler to analyze than the video output signal of an area scan camera, most web inspection systems utilize a line scan camera, rather than an area scan camera, as the mechanism to scan the moving web of material. Web inspection systems employing a line scan camera are shown, for example, in U.S. Pat. Nos. 4,724,481, 5,068,799, 5,132,791 and U.S. Pat. Re. No. 33,357.
Most web inspection systems utilize some type of digital signal processing techniques to analyze and detect defects in the video output signal. Typically, a video output signal from a CCD camera is supplied to an analog-to-digital (A/D) converter so as to generate a stream of digital bits representative of the video image of the web of material moving past the CCD camera. The use of digital signal processing to analyze and detect defects has made web inspection systems more adaptable to a wider variety of web inspection problems and, in most cases, has increased the accuracy of the web inspection process.
Prior to supplying the video output signal to the A/D converter, it is helpful to filter the raw video output signal using a preprocessing circuit. Preprocessing circuits are used to enhance the effectiveness of the A/D converter by filtering the raw video output signal to produce a filtered video output signal. In most cases, the preprocessing circuitry will remove a large DC bias offset that is part of the raw video signal and will combine separate video streams from the odd and even photosites in the scan array into a single filtered video signal. In many instances, preprocessing is performed internal to the CCD camera such that the raw video signal from the scan array is not even available as an output signal from the CCD camera. In higher quality CCD cameras of the type used for web inspection systems, the raw video signal, as well as a filtered video signal, are typically available as outputs from the CCD camera.
The problem with digital signal processing of the video signal in a web inspection system is that the digital signal processing can only be as accurate as the accuracy of the A/D conversion of the original video signal. In the case where video preprocessing circuits are used, the accuracy of the A/D conversion is necessarily limited by the accuracy of the video preprocessing circuit. Unfortunately, existing video preprocessing circuitry for line scan cameras can lose valuable signal information and may actually introduce additional error into the video signal.
The consequence of introducing errors into the video output signal are particularly significant for a web inspection system. Unlike a video signal from a VCR camera, for example, where the video signal is intended to be viewed as an entire picture, a video signal that is used by a high performance web inspection system may be analyzed on a pixel-by-pixel basis. While an occasional glitch or light spot in a VCR recording will most likely go unnoticed by the viewer, even a single pixel error or variance can result in the detection of a false defect, known as a false positive. If there are too many false positives generated by the web inspection system, the result can be an incorrect rejection of huge amounts of web material.
As an example, high performance web inspection systems typically scan a jumbo of rolled web material that is 7315 m (8000 yards) long and 1.52 m (60 inches) wide. In this example, if the web inspection system is attempting to detect defects that are as small as 2.5 mm.sup.2 (1/16inch.sup.2), there will be a total of 4,400,000,000 potential defect sites in a jumbo that need to be scanned. If the defect rate at which the entire jumbo will be rejected is set at 5 defects/jumbo, then a web inspection system which is configured to check for defects as small as one pixel must consistently distinguish actual defects from false defects with a defect detection consistency rating of no more than one pixel variance/billion pixels scanned.
Regardless of what type of digital signal processing techniques are used, if the underlying video output signal provided to the A/D converter is itself not as consistent as the desired consistency rating, then there is no way to improve the defect detection consistency of the overall web inspection system. Consequently, a web inspection system that uses a CCD camera having enhanced video signal preprocessing capabilities would be greatly appreciated.